Change code to remove two warnings that are reported when running build.py.

site/try/src/interaction_manager.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library trydart.interaction_manager;
 
 import 'dart:html';
 
 import 'dart:convert' show
     JSON;
@@ skipping 744 matching lines @@
     context.compilerConsole = new SpanElement()
         ..append(progress)
         ..appendText('\n');
     context.shouldClearConsole = true;
     context.elapsedCompilationTime
         ..start()
         ..reset();
     if (context.isFirstCompile) {
       outputDiv.append(context.compilerConsole);
     }
+    var diagnostics = mainEditorPane.querySelectorAll('a.diagnostic');
     context.oldDiagnostics
         ..clear()
-        ..addAll(mainEditorPane.querySelectorAll('a.diagnostic'));
+        ..addAll(diagnostics);
   }
 
   void aboutToRun() {
     context.shouldClearConsole = true;
   }
 
   void onIframeError(ErrorMessage message) {
     // TODO(ahe): Consider replacing object URLs with something like <a
     // href='...'>out.js</a>.
     // TODO(ahe): Use source maps to translate stack traces.
@@ skipping 564 matching lines @@
     token = token.next;
     kind = token.kind;
   }
   return token;
 }
 
 String extractQuote(String string) {
   StringQuoting q = StringValidator.quotingFromString(string);
   return (q.raw ? 'r' : '') + (q.quoteChar * q.leftQuoteLength);
 }